Method of forming refractory molds for metal casting



Patented June 29, 1954 METHOD OF FORMING REFRACTORY MOLDS FOR METAL CASTING John A. Henricks, Lakewood, Ohio" No Drawing. Application May 6,1950,

Serial No. 160,577

3 Claims.

This invention relates to the casting of metals and to the treatment of molds for metal casting and is a continuation-in-part of my copending application Serial No. 666,521, filed May 1, 1946, now abandoned.

For purposes of illustration the invention is disclosed herein principally as applied to the art of precision casting by the lost wax or investment method, its application to other methods of casting to provide greater precision and better surface qualities of castings and better refractory qualities of molds being readily apparent from the illustrative examples.

In the prior art of precision casting by the lost wax or investment method, a temporary pattern is formed of wax and is invested with a plastic refractory composition which is allowed to set, after which the investment with the ineluded wax pattern is subjected to heat in a furnace to dry the investment and melt the wax so that it can be drained out of the investment. Subsequently, the metal to be cast is poured into the mold cavity thus formed in the investment.

Several disadvantages result from this practice. First, raising the temperature of the investment and invested wax in a furnace so as to melt a substantial portion of the wax to fluidity results in the molten wax being absorbed in the interstices of the investment of capillary action. Consequently, a temperature higher than that required to melt the wax must be applied and maintained for a sufficiently long period to burn out the wax so absorbed, for example, temperatures as high as 1000 F. maintained for a period of four to eight hours. This step, known as the burn-out step, necessitates in turn the use of a relatively ash-free and expensive wax, and a rather large and expensive furnace, and also the raising of the temperature of the refractory above a temperature at which important phase changes occur. Again, the wax has a much greater cofficient of expansion than the refractory at such higher temperatures and during the burn-out step there is a tendency for the expanding wax to crack the surrounding mold or investment. In addition, due to the absorption of the liquid wax into the pores and interstices of the refractory investment as the wax melts, a relatively high percentage of wax is lost during each burn-out. Another disadvantage resides in the fact that the wax absorbed and thus reduces the porosity of the refractory investment so that venting of the gases during subsequent metalcasting is inefiicient and diincult. Finally, the investment walls exposed to the molten. metal during subsequent casting are unprotected and tend to be roughened or eroded by the wash of the molten metal.

One of the objects of the present invention therefore is to eliminate the burn-out step heretofore required in the lost wax method of precision casting, and the disadvantages resulting therefrom.

Other objects are to increase the wax recovery, to reduce thetimerequired for the preparation of refractory molds for subsequent casting, to eliminate the necessity for large and expensive furnaces heretofore required, to reduce time and space requirements, to make possible the use of relatively inexpensive waxes and fillers instead of the expensive'ash-free waxes heretofore found necessary,'to increase the refractory nature of the binders used in the investment, to inhibit chemical action between the refractory molds and the cast metal, to' prevent the formation of oxide'films on'the surface of molded metal, and to provide refractory molds of sufficient porosity to permit'the escape of gases during casting.

Another object'is 'to'render possible the precision casting of ferrous metals by the lost wax method.

Other objects and advantages will become apparent from thefollowing description wherein the invention is described principally as applied to precision casting'by the lost wax or'investment method. ii

Briefly, in accordance with the present invention, the wax pattern is formed in the usual manner, though a less'expensive wax, or wax and filler, may be used instead of the relatively ex pensive ash-freewax heretofore required.

Investment casting waxes are formulated by adding hard natural waxes such as carnauba or candelilla to a cheaper petroleum wax base. Shellac andvarious natural or synthetic resins, as well as heavy metal soaps and fatty acid amides are also used as hardening agents in such formulations. The term wax as used herein, is meant to include thewaxes and wax compositions customarily used for such investments.

Next, the investment of the refractory and binder composition is made in the usual manner except for the new step of rendering the binder insoluble in water, either by direct chemical action or by treatment to cause secondary reactions with the aqueous medium, and concurrently increasing its refractory characteristics.

As next and novel step, after the investment is poured about the pattern and the insolubilized binder has set, the investment, with the wax pattern included therein, is placed in a hot aqueous medium for a sufiicient period to melt out the wax from the investment. After the wax has been melted out in the aqueous media, the investment is dried in an oven to remove the water and open up the pores. This dry-out step requires from one to two hours at temperatures usually not exceeding 500 F. the burn-out step of the prior art required about twice as much time at a temperature of about 1000 F. As a result of this change in the removal of the wax from the investment, no wax is absorbed into the mold interstices by capillary action and a great saving in wax recovery is efiected. Furthermore, the time required for preparation of the mold after investment of the wax pattern is greatly reduced and the equipment necessary is not as great and is much less expensive inasmuch as a core drying oven can be used instead of a high temperature furnace. The flasks used can be the simple nonscaling chromium steel instead of "Inconel or other high priced alloys.

An additional advantage obtained by an oven dry-out at temperatures less than 500 F. is that this avoids the important phase changes in the various refractory materials which occur above 500 F. For example, the rapid inversion of the alpha quartz characteristic of most sands to the beta quartz, tridymite or cristobalite, does not occur below my optimum drying-out temperature, so that extreme mold shrinkage or expansion is eliminated and the dimensions of finished castings can be controlled more accurately.

By utilizing aqueous media, the pores and interstices of the mold take up the aqueous media by capillary action instead of absorbing the wax, which .is of a hydrophobic nature and is repelled by water. By melting out the wax in aqueous media, the expensive and critical burn-out operation is eliminated and the expensive pattern wax, which is not miscible with water, can be recovered fully, and readily reclaimed and reused. Instead of de-waxing in aqueous medium, the mold and the invested wax can be treated in the medium below the melting point of the wax to insolubilize the binder and saturate the mold with water, and the Wax can subsequently be melted out by steam.

As the next novel step, the surface of the mold cavity is impregnated with a fusible flux film so thin as not to change dimensions of the cavity and capable of chilling a thin skin of the metal next to the mold wall to protect the mold and prevent oxide inclusions in the metal. Thereafter, the mold is dried and can be poured in the usual manner. The fusible flux not only acts to chill a thin skin of metal against the mold face and to flux out oxides, but aids the metal to wet the mold face thoroughly to assure exact duplication of the finer details of the pattern.

It is preferred, in accordance with the present invention, to provide a shrinkage riser in the mold and vibrate the mold while maintaining the metal in the shrinkage riser in molten condition by the application of heat by means of an electric arc, induction heat, or melting torch. Vibration may be effected mechanically by means of a motor-driven or (SO-cycle solenoid vibrator. This manner of pouring assures complete filling of the mold cavity.

While binders such as plaster of Paris (calcium sulfate) or the hydraulic cements (impure calcium silicates or aluminates) are only affected to a small degree by water after their initial set, the use of alkali metal silicate, phosphate, or oxychloride cements as binders requires the insolubilizing action of certain chemical agents to prevent complete disintegration of the refractory investment during the melt-out of the pattern wax in an aqueous medium. The insolubilizing action of these chemical agents upon the adhesives or cements used to bind the refractory powders of the investment comprises a very important part of this invention. Besides making de-waxing in aqueous media possible in the case of those silicate, phosphate, or oxychloride cements that would otherwise tend to disintegrate in water, the chemical treatment of the binder can be directed both to increasing the refractory nature of any of the binders and to inhibiting any chemical action between the binders or cements and the molten metal being cast.

In order to eliminate mold erosion during aqueous dewaxing, the ion exchange reagent is brought closer to the neutral point by the addition of a mildly alkaline solution such as sodium phosphate or carbonate to an acid salt such as aluminum or zinc chloride, or by the similar neutralizing action of an acid salt such as magnesium or chromium chloride to the alkaline reagents such as sodium aluminate or sodium silicate which in each case, not only brings the reagent closer to the neutral point, but also forms adhesive and colloidal metal hydroxides which augment and protect the initial inorganic binder.

As an instance of the step of insolubilizing by a secondary chemical reaction a binder such as sodium silicate (Na2O3.2SiO2), the dry set-up mold bound with this adhesive is placed in an aqueous solution of one pound per gallon of ZnClz and the solution raised gradually from room temperature to its boiling point, by which time the wax will have been eliminated from the cavity, and the ZnClz will have reacted with the sodium silicate to insolubilize it by the formation of zinc silicate. This insolubilizing step also improves the refractory characteristic of the binder, the zinc silicate having a melting point of 2750 F. whereas the sodium silicate has a melting point of about 1525 F.

As another instance of indirect solubilizing, when a binder of Portland cement is used for cementing the refractory, the dried mold may be immersed in an aqueous solution of 12 ounces per gallon of sodium aluminate which, upon bringing the solution to boiling and maintaining it at such temperature for a sufficient period to melt the wax, causes the formation of calcium aluminate by reaction with the free lime of the cement. Here again, the insolubilization increases the refractory characteristics of the mold.

Heretofore it has not been clear why indifferent results and surface irregularities were obtained when the dental or jewelry techniques were applied to the industrial precision casting of iron, nickel, or cobalt alloys. The gold and platinum used in jewelry and dentistry are noble metals and chemically inert unlike molten iron which, for example, will react with the silica of a hot refractory mold. F. H. Norton in Refractories (McGraw-Hill, 1st ed. 1931) gives some interesting figures on page 380 which show the reaction of metallic iron and iron mill scale upon the common refractory materials. The data were obtained by air spraying test cylinders of the various refractories with metallic iron or i TABLE II. PERCENTAGE.

mill scale while the specimens were held in a furnace at the temperature shown. The volume of TABLE I. PERCENTAGE RESISTANCE TO METALLIC IRON Percent Resistance Melting RESISTANCE OF REFRACTORIES- TO MILL SCALE;

Percent Resistance WWWWlltlllMinimum These tables indicate the essential chemical nature of the reactions between the metal and the refractory and that resistance is not a mere matter of the melting point of the refractory. It is to be noted that neither aluminum nor silicon oxides show any resistance to metallic iron at 2850 but that when they are combined chemically to form aluminum silicates such as mullite or kaolin they show resistance. The low resistance of kaolin to mill scale at high temperatures is due to the flux action of the iron oxides upon the clay, while the lowered resistance of the silicon carbide to mill scale is due to carbide oxidation by the ferric oxides, both being chemical reactions. It is to be noted that silica, which is used as the refractory for most investments because of its high expansion under heat, is badly attacked both by metallic iron and by iron oxides. Spinel dissociates at high temperature into magnesium oxide and aluminum oxide then reacting with metallic iron like plain bauxite to account for the attack of iron on spinel at 2850 F. Likewise, the uncombined silica of the kaolin dissociates at 2850 to increase the reaction rate of kaoline with metallic iron at this temperature.

Since both iron oxides and metallic iron are in contact with the refractories when the castings are made, one would conclude from Nortons data that magnesia would be the only safe material for use with ferrous alloys at 2850 F. While the basic oxides such as lime or magnesia have little chemical reaction with ferrous metals, they are weak and chalky unless vitrified by burning at about 3200 F., and in addition have a tendency to carbonate in storage and when made up into molds and heated in a furnace. The calcium or magnesium carbonates so formed break down in contact with molten metal to deface the casting with gas pockets.

I am able to overcome the deleterious reac- NU! coooooomo:

tion during casting between the refractory and the metal or its oxides by the methods disclosed in this invention. By these novel means the refractory grains are coated with a cement or binder so that they have no direct contact with the metal, and the cement or binder is then rendered inert and more highly refractory by chemical treatment in aqueous media.

Chemical reaction between the mold and the molten metal is a key factor that must be carefully controlled to obtain a good finish on the poured casting. Plaster of Paris (calcium sulfate) can be used as a very satisfactory investment binder for brass or aluminum castings which are cast below 1450" F. but cannot be used in accordance with prior methods for ferrous castings which are poured above 2000 F. because of a strong reaction between the iron and the sulfate radical causing the formation of iron sulfide and a pitted unacceptable finish. Likewise, when a ferrous metal is poured into a hot mold made up of silica powder cemented with a silicic acid binder, a burnt-in skin or scab is formed which spoils the casting finish. Chemically, a molten metal behaves as though it were an activated ion in solution. Thus the breakdown reaction (designated as Reaction 1) of the ferrous metal and sulfate in the plaster mold is one in which the ferrous metal, the positive radical, and the sulfate, the negative radical, react to form ferric oxide, ferrous sulfide and ferrous oxide. The burning in of the ferrous metal in the silicic acid bound mold could be expressed in this reaction:

(2) FB+H2SiOs- FBSiOs-l-Hz In order to eliminate the reaction between iron and sulfate shown in Reaction 1, the plaster bound mold can be treated with, for example, a solution of sodium aluminate, giving the following reaction:

(3) CaSO4+NazAl2O4 Na2SO4+ CaAl2O4 (insoluble) a reaction with, for example, a solution of zinc chloride.

The insolubilizing chemical treatments used to decrease the reactivity of the casting metal With the mold binder can be grouped into three general classifications.

In the first class, the cement or binder contains sodium, ammonium, potassium, hydrogen, or other soluble cations which will exchange with a polyvalent cation in aqueous solution to form an insoluble, more refractory binder which will be inert to the metal being cast.

The amphoteric metal salts of metals capable of forming refractory oxides; for example, metals such as aluminum, zinc, chromium, titanium and zirconium (as distinguished from low melting metals which are not refractory such, for example, as lead, tin and antimony), are the preferred type of polyvalent cation. These salts form adhesive colloidal hydroxides during the transition period between the acid metal salt and the soluble alkaline zincate or aluminate which helps to reinforce the initial binder when the molds are transferred from acid to alkaline reagents during treatment. Further ion ex- 2,682,092 7 8 change can be provided in the flux treatment, as The residual zinc chloride is left in the mold to shown in the following example. form a fusible flux film. The following table will In the case Where a partial wax recovery is clarify the nature of the ion exchange mechanot desired, by means of dewaxing in the salt nisms:

CURING REFRACTORY BINDERS IN AQUEOUS MEDIA V (l) CEMENTS CONTAINING SOLUBLE CATIONS Type of Binder Type of Solution Used in Cure Aqueous solutions of the chlorides of chromium, magnesium, aluminum, zinc, barium, iron, etc. The soluble sulfates or acetates of the polyvalent metals are also Boric acid H 303 Satisfacwry- Alkali metal borate (NazoBzoofII:IIIIIIIIIIII (2) OEMENTS CONTAINING SOLUBLE ANIONS Magnesium oxychloride (MgzOClz) Zinc oxychloride (ZngOClz) Aqueous solution of an alkali metal chromate, aluminate, zincate, phosphate, or sill- Calciuin sulfate (CaSO4)..III::::::::::::: cate, or solutions of phosphoric or silicic acids.

(3) MISCELLANEOUS ION EXCHANGE MECHANISMS Plaster of Paris (02180 Solution of monosodium phos- Solution of alkali metal silicate, borate, aluminate or phosphate.

Solution of alum or barium hydroxide.

solution of an amphoteric refractory metal, or where the wax pattern is burned-out and the mold brought up to temperatures above 1660 F. for casting thin sections, an ion exchange reagent can be added to the silica sol formed by adding diluted water glass to a diluted strong acid solution so that ion exchange will occur during drying and heating up the mold. The use of a salt solution of an amphoteric refractory metal in the binder also eliminates the necessity of such a polyvaient cation treatment in the cycle.

In the second class, the cement or binder contains a soluble anion, such as chloride, or sulfate, which will exchange with a ceramic anion in aqueous solution to form a less soluble, more refractory binder which will be inert to the metal being cast. The term ceramic anion is used hereinafter to designate the glass-forming aluminate, boratc, phosphate, silicate, zirconate, chromate, zincate, or titanate anions.

When a refractory of the highest order is required, or when the flux impregnating solution is chemically reactive with the binder insolubilizing reagent, reactions of both the first class involving cation exchange and the second class in- VOlViIlg anion exchange can occur. For example, a gypsum bound mold can be rendered insoluble and inert by anion exchange with sodium silicate:

It should be pointed out that the above ion exchange reactions need not be fully completed to achieve the desired increased refractoriness or inertness to the metal being cast. As long as an insoluble skin is formed over the binder by an ion exchange, so that the binder will not react with water during the aqueous dewaxing of the mold, or react with the casting metal in pouring, satisfactory results are obtained. In order to prevent the rupture of such a skin formed on the cement or hinder by ion exchange, any salt concentration used in the aqueous media should be higher than the salt concentration of the binder, because if a dilute aqueous solution of the insolubilizing reagent is used, the thin precipitated membrane is apt to be ruptured by imbibing water by osmosis to equalize the two salt concentrations and to reduce the resulting osmotic pressure. If greater penetration of the aqueous media used to increase the binder insolubility or refractoriness is desired, it may be attained by adding a wetting agent to the reagent solution to lower the surface tension of the water. A still greater penetration of the aqueous reagent can be obtained by drying out the dewaxed mold at a temperature above 212 F. to remove all the Water from the pores of the mold so that it will be capillarily active and thoroughly imbibe the treating reagent. In some cases it may even be (5) desirable to drive oil the bound water of crystal- 0380421120 music; NazSOrl-CaSiOa(inSOluble)-I-2Hz0 lizatioll in the binder to further p p the (Soluble, (reagent) (dissolves (soluble 0mg mold. Such dehydration of the mold allows the g lgg Pm 100) aqueous ion exchange reagent to penetrate the water) mold completely rather than to merely form a.

The initial treatment leaves an absorbed film of sodium silicate within the mold which is then rendered inert and insoluble by cation exchange by a zinc chloride flux solution, as follows:

(soluble) (reagent) ZNaCl (dissolves in reagent) ZnSiO:

(insoluble) vestment powders to eliminate the extreme mold shrinkage which takes place around 700 F. when the last of the bound water in the calcium sulfate is given off. Such additions, however, modify the normal setting time of the calcium sulfate and reduce the mold strength, and are undesirable for these reasons. By the novel methods of this invention these drawbacks can be overcome. The silica and plaster of Paris are mixed with water and allowed to set up to full strength in their normal manner, after which the set mold can be impregnated with a solution of boric acid or a chloride to get a higher mold expansion. A greater uniformity is obtained if the mold is first dewaxed and dehydrated before such impregnation.

The following examples illustrate insolubilization of the binder:

Example I.CoZZoidaZ silica binder When a number of different metals and different types of patterns are to be cast, I prefer to use a universal investment for all molds, and then to precoat and flux those which require special treatment. An illustration of such a universal investment is:

Pounds 325 mesh silica flour 8 90 mesh silica flour 12 80 to 100 mesh banding sand 20 Zinc oxide (accelerator) Liquid A:

250 cc. 85% phosphoric acid diluted with 750 cc. water (=3 mols of H3PO4).

LiquidB:

600 cc. of 38%-Na2O'3.2SiO2 water glass (:4 mols S102) 2400 cc. of water containing oz. of a nonionic wetting agent (such as polyethylene glycol stearate and about cc. of a defoamer (e. g. octyl alcohol).

The silica powders are first thoroughly blended, and then the liquids are made up. Since 85% phosphoric acid has a considerable heat of di1utized by the excess phosphoric acid to form a 001- r loidal binder. This reaction also gives off heat, and the mixer should be cooled for production mixing. The investment can be mixed in a cement mixer, a cake type mixer, or foundry sand muller, silica powder in each case being sifted into the mixed cool liquids.

It is important that the mixed investment material be kept in agitation during the investing, in order that the coarse particles do not settle out during the flask filling cycle. The poured investment does not require a vacuum treatment because of the wetting agent present in the liquid, but it must be tamped or vibrated around the wax pattern. The usual tamping table equipment is suitable, but any 60 cycle electrical or mechanical vibrator is satisfactory to pack the wet investment around the pattern. A three inch gummed paper collar is put around the top of the flask or an overlength paper is used as a liner, so that the excess silica gel can rise above the flask proper to eliminate shrinkage. After the investment has set up, the excess collar above the flask is cut oi! to leave a dense, homogeneous flask of investment. The flask should preferably set over night for best results, but it can be dcwaxed in an hour or two after the collar is out off. If a faster cycle is required, the formation of the above binder can be varied through the following ranges for a pound mix:

The maximum concentration is set by the tendency to form too much silica gel, which causes mold cracking due to silica gel shrinkage when dried. The minimum concentration is determined by the loss of strength which in the case of (3) above results in a fragile mold that needs an addition of 1 to 2% of an organic binder, such as dextrin or gum arabic to give it sufficient working strength, or this weak mold may be dewaxed in a solution of a ceramic binder, then oven dried for strength. The silica sol formed by neutralizing the free alkali of the water glass is too unstable if the neutralization goes beyond the formation of monosodium phosphate and the pH goes above pH 3.0. The most stable silica sol is formed when suilicient excess acid is used to have one moi of NaH2PO4 and one moi of H3PO4 or equivalent strong acid so in equilibrium, that the pH is kept between 1.0 and 2.0.

The function of the free acid is to peptize the silica sol, so that a minimum acid such as shown in (4) above will set up in half the time of the preferred amount in Example I, as will be the substitution of a. weak acid such as acetic acid for the phosphoric acid shown.

After the molds made according to this method had set up gelation of the silica sol, the flasks were trimmed and placed in a 5 to 10 ounces per gallon soluble aluminum acetate solution, with the sprue end up, and the solution brought up to boiling. During this heating cycle, the large bulk of the pattern wax is displaced by the heavier aqueous solution and floated on the surface of the solution, and the pores of the mold saturated with aluminum acetate solution. The treated mold is drained, and, if desired, rinsed with hot water, and then immersed in a warm alkaline zincate hath made by adding a 10% zinc chloride solution to a 10% sodium hydroxide solution with stirring so that the initial precipitate is redissolved. This alkaline zincate bath removes the residual wax from the mold cavity by saponification and emulsification. When the mold is completely dewaxed, the alkaline solution is drained off and the mold cavity rinsed out and treated with 10% hydrochloric acid to neutralize the alkaline residue and leave a fiux film containing free acid as well as zinc, aluminum and sodium chlorides.

The ion exchange mechanism in this cycle consists of the formation of aluminum silicate and phosphate by exchange between the aluminum acetate solution and the silicic and phosphoric acid of the binder, followed by colloidal alumina formation when the aluminum acetate residue reacts with the alkaline detergent reagent which also would form an insoluble refrac- 1' 1? tory zincate with any labile polyvalent cations in the mold.

Example II.--Colloidal silica binder containing salt of refractory amphoteric oxide (1) Dry investment powder:

20 pounds of 325 mesh amorphous silica 30 pounds of 140 mesh ground quartz 30 pounds of 80 mesh banding sand Mixed into uniform powder blend. (2) Silicic acid sol binder:

Solution A- 717 ml. of 85% H3PO4 950 ml. of Water The acid is diluted with water and cooled. Solution B 1667 ml. of 38%--NazO-3.2SiOz water glass 3333 ml. of water The water glass is diluted with water. Solution C 3333 ml. of 27% ZnClz solution As in the previous example, the "diluted water glass is run into the diluted phosphoric acid with energetic stirring to form "a silicic acid sol, after which addition, the zinc chloride solution is added to the silicic acid-phosphoric acid mixture, also with stirring.

The investment powders are then added to the aqueous binderliquid, usinga concrete or other appropriate mixer.

The mixed binder and silica investment is used to invest wax patterns in the same manner as Example I. If so desired, these molds can be heated up to above 1000? F. to burn out the wax pattern and form an additional ceramic bond by fusion of the residual monosodium phosphate with the refractory, and the metalthen cast into the hot mold.

The set up molds are otherwise dewaxed in a solution of 4 to 8 ounces per gallon commercial sodium aluminate solution; which removes the wax patterns by saponifi'c-ation and emulsification, after which the mold'cavity is rinsed with hot water and impregnated with a10%' solution of zinc ammonium chloride; and the mold dried out preparatory to casting;

The ion exchange mechanism in this cycle begins with reaction between the zinc cationand silicic and phosphoric acid during 'the binder set-up period, followed by formation of colloidal zine hydroxide and zinc aluminate during the sodium aluminate reagent treatment, and is completed when the zinc ammonium chloride forms additional colloidal alumina-and zinc aluminate.

Colloidal alumina and alumina-tea are also excellent binders that are also well adapted to become more refractory and inert to' the metal being cast as shown in the'followingexample.

Example IIL-Qolloidal alumina binder (1) Dry investment powder:

7% commercial calcium aluminate cement 18% 325 mesh amorphous-silica 25% 140 mesh ground quartz. 50% 80 mesh banding sand An investment is made up by mixing 150 ml. of 10% A1C13'6H2O per pound of investment powder, and wax patterns invested in the usual manner. The aluminum'chloride not only furnishes colloidal alumina by reaction with the alkaline cement, but its acidic nature destroys certain harmful sulfide residues in the commercial aluminous cement. A 2 to mono- 12 ammonium phosphate solution can be used in place of the aluminum chloride for the same purpose, and produce a good investment although the aluminum chloride is preferred for ion exchange treatment.

When the aluminous binder investment has set up, the bulk of the wax pattern is removed from the mold cavity by heating up the mold in a 5 to 10% solution of chromium acetate, after which the partially dewaxed mold is treated in a detergent solution of 5% sodium metasilicate to remove the balance of the wax by saponification and emulsification.

After the Wax pattern had been removed, the mold cavity was rinsed out and a 5% solution Of phosphoric acid used to neutralize the alkaline aluminate residue and leave a sodium and aluminum phosphate flux residue.

The ion exchange mechanisms in this cycle begin with the formation of chromium aluminates, followed by precipitation of colloidal chromium hydroxide, and subsequent formation of chromium silicate. The phosphoric acid flux will also precipitate colloidal silica which would form aluminum silcates during the mold drying.

These examples illustrate the various ion exchange mechanisms used to obtain investment molds that will be inert to the metal being cast, and have a higher degree of refractoriness than could be provided by the colloidal binder alone. The use of salts of amphoteric metals forming refractory oxides is an important part of this invention, since it will lead to spinel formation between the separate reagents under heat, so that no reactive oxides are left in the mold after multiple reagent treatment. For example, if zinc chloride is used either as a flux or a dewaxing medium, it will be combined with sodium aluminate treatment to leave a refractory zinc aluminate spinel residue.

The novelty of dewaxing an investment mold in aqueous media to avoid the difiiculties of refractory cracks brought on by a long burn-out at a red heat, and the novelty of curing the mold binder by specific ions that produce a cement which is of a highly refractory nature and is indifferent to the metal being poured, as disclosed in this specification, combine to produce precision castings superior to those obtainable in the prior art.

Structural surface weakness in the finished casting sometimes encountered can be traced to oxide inclusions in the casting skin. Ths condition can be avoided, in accordance with this invention, by the cautious use of a thin fusible film of a metal flux on the face of the mold cavity. After first dewaxing and curing the mold to produce an inert and refractory binder and Washing out the chemical treating reagents with water, an aqueous dilute solution of the specific flux is poured into the mold cavity and allowed to soak in for a few minutes, and then the residue poured out and the mold then is dried in an oven to remove all the water and, if desired, further heated up to an elevated temperature for pouring. This results in a thin film of fusible flux on the cavity wall. While a saturated solution or undissolved fiux powders are normally used for deoxidation of metals in such operations as soldering or brazing, it is preferred to use herein a relatvely dilute solution that leaves such a thin film of fusible fiux on the mold cavity face that it will not change the precise dimensions of the finished casting or produce any boiling action when the metal is poured. Also, mold porosity 13 must be maintained and not lost as would be the case with any heavy film of the dried fiux in the mold interstices. Again, the dilute aqueous solution of. flux may be included in the aqueous media or bath usedfor dewaxing. The following table shows some specific wash fluxes and their use:

MOLD ASH FLUX The function of the fusiable flux film in this novel precision casting method is somewhat analogous to the function of moisture in green sand casting. As pointed out above, a molten metal acts chemically as an active metal ion in solution. In green sand casting the steam acts as an acid with the molten iron:

The nascent hydrogen thus produced has a reducing action on the chilling metal to limit greatly the oxide inclusions in the surface skin. The cooling action of the water is high, due to the high latent heat of vaporization resulting in the steam formation. This chilling action is very important since it forms a thin skin that protects the fragile sand from the wash of molten metal. In the present invention, the fusible flux film is used which likewise chills a thin skin of metal against the mold face to protect the mold against the wash of the heavy molten metaL. The thin film of minute flux crystals is fused by the initial contact with the molten metal, which fusion in turn chills the metal to form a thin protective skin of metal against the mold face to protect it against thermal shock or burn-in, as well as against metal wash. The fused flux film dissolves any oxide formed by contact of the molten metal with air, so that the very important skin on the casting is oxide-free and mechanically and metallurgically sound.

In precision casting by the lost wax process, the patented and commercially used investment binders can be placed in four general groups according to their chemical properties.

Example IV.Treatment of oxyphosphate cement Zinc and magnesium oxyphosphate cements are widely used in the dental art. A characteristic refractory investment of this type is given in U. S. Patent No. 2,209,035 to Emil M. Prosen:

Parts Silica (carefully graded sizes) i 94 Magnesium oxide 6 Monobasic ammonium phosphate 3 Monobasic sodium phosphate 1 The above dry powder mixture was made into a thick cream with water and a wax pattern invested in the usual manner. Since the above formula has about three times the equivalent magnesia necessary to neutralize the acid phosphates, an anion is required to react with the excess magnesia base. In this case, a solution made up of one part of Na2O.3.9SiO2 water glass (30% solids) diluted with two parts of water was used as a reagent to react with the free magnesia and increase the strength of the mold. The mold was dewaxed and rendered inert in a sodium silicate solution. The silicate treated mold was drained, rinsed, and immersed in a 5% boric acid solution. The boric acid solution was used to precipitate any residual sodium silicate as a silicic acid gel in the pores of the mold, and to deposit a flux film in the refractory mold. The boric acid impregnated mold was dried and oentrifugally cast with stainless steel as in the previous examples. The resultant casting was easily cleaned and had a superior finish.

If a magnesium phosphate binder having an excess of phosphate were to be used for such a mold, a soluble divalent base such as barium hydroxide would be required to insolubilize the excess phosphate ion, instead of using a silicate to insolubilize the excess magnesium ion present in the Prosen composition.

I have noticed that the oxyphosphate binders have a higher strength if they are dried out before being treated with the insolubilizing media. In order to dry the mold properly, it is first dewaxed by boiling in a 1% to 5% solution of magnesium or zinc chloride, drained and rinsed, then oven dried for two hours at 200 F. to 300 F. The dried-out mold is then immersed in the sodium silicate reagent in the same manner as the above example. It is believed that the higher strength is a result of better penetration of the glass forming anions into the interstices of the mold, whereby the precipitated magnesium or zinc salts tie the refractory together by interlocking crystals similar to those of plaster or cement binders.

It is to be understood that the zinc or copper oxyphosphates have the same chemical characteristics as the magnesium oxyphosphate binders and can be rendered inert and insoluble by similar treatments.

Magnesium and zinc oxide also form cements with strong solutions of magnesium and zinc chloride. The oxychloride cements have poor refractory properties and heretofore have not been used for high temperature work. By utilizing the novel ion exchange methods of this invention, the unstable chloride ion is exchanged for ceramic anions so that satisfactory heat stable binders can be obtained.

Having thus described my invention, I claim:

1. In the preparation of molds for the casting of metals by the lost wax method, the steps of forming an investment of refractory powder and an inorganic colloidal binder, said binder being a material selected from the group consisting of acidulated alkaline aluminate, acidulated alkaline silicate and cementitious acid phosphates; investing a wax pattern with said colloidal binder and refractory powder; allowing the investment to set to form a mold; subjecting the formed mold containing the invested pattern therein to a hot aqueous solution of an acid salt of a polyvalent metal to melt out and physically displace the bulk of the wax in the mold cavity and to cause the displaced wax to salt out in readily recoverable form, said polyvalent metal being selected from the group consisting of zinc, aluminum, chromium, titanium and zirconium, said hot aqueous salt solution reacting by ionic exchange with the binder of said investment to increase the refractoriness of the mold; then removing any residual wax remaining in said mold and cleaning the surface of the mold cavity by applying an aqueous solution of a detergent to said mold with said detergent solution reacting by ionic exchange with the product formed by the reaction between said binder and said hot aqueous salt solution to further increase the refractoriness of the mold; the solute of said detergent solution being a material selected from the group consisting of alkaline aluminate, alkaline zincate, alkaline zirconate, alkaline titanate, and alkaline silicate; and then heat drying the mold.

2. In the preparation of molds in accordance with claim 1 the further steps of applying to the surface of the mold cavity an acidic flux in liquid form for the metal to be cast and then drying the mold.

3. The preparation of molds in accordance with claim 2 characterized in that the flux is material acidified by acid selected from the group consisting of hydrochloric acid, phosphoric acid and boric acid.

References Cited in the file of this patent Number UNITED STATES PATENTS Name Date Pack et a1. Apr. 16, 1918 Beatty Jan. 18, 1927 Laukel Feb. 16, 1932 Prange Ma 16, 1933 Sebrell May 8, 1934 Ray Jan. 14, 1936 Moosdorf et al. Mar. 2, 1937 Verwey et a1. June 8, 1943' Bull Aug. 24, 1943 Schoonover et a1. Nov. 9, 1943 Bean Dec. 25, 1945 Whitehead Feb. 10, 1948 Jeter et a1. June 1, 1948 Frei Sept. 20, 1949 Barr Dec, 6, 1949 al-A 

1. IN THE PREPARATION OF MOLDS FOR THE CASTING OF METALS BY THE LOST WAX METHOD, THE STEPS OF FORMING AN INVESTMENT OF REFRACTORY POWDER AND AN INORGANIC COLLOIDAL BINDER, SAID BINDER BEING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ACIDULATED ALKALINE ALUMINATE, ACIDULATED ALKALINE SILICATE AND CEMENTITIOUS ACID PHOSPHATES; INVESTING A WAX PATTERN WITH SAID COLLOIDAL BINDER AND REFRACTORY POWDER; ALLOWING THE INVESTMENT TO SET TO FORM A MOLD; SUBJECTING THE FORMED MOLD CONTAINING THE INVESTED PATTERN THEREIN TO A HOT AQUEOUS SOLUTION OF AN ACID SALT OF A POLYVALENT METAL TO MELT OUT AND PHYSICALLY DISPLACE THE BULK OF THE WAX IN THE MOLD CAVITY AND TO CAUSE THE DISPLACED WAX TO SALT OUT IN READILY RECOVERABLE FORM, SAID POLYVALENT MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF ZINC, ALUMINUM, CHROMIUM, TITANIUM AND ZIRCONIUM, SAID HOT AQUEOUS SALT SOLUTION REACTING BY IONIC EXCHANGE WITH THE BINDER OF SAID INVESTMENT TO INCREASE THE REFRACTORINESS OF THE MOLD; THEN REMOVING ANY RESIDUAL WAX REMAINING IN SAID MOLD AND CLEANING THE SURFACE OF THE MOLD CAVITY BY APPLYING AN AQUEOUS SOLUTION OF A DETERGENT TO SAID MOLD WITH SAID DETERGENT SOLUTION REACTING BY IONIC EXCHANGE WITH THE PRODUCT FORMED BY THE REACTION BETWEEN SAID BINDER AND SAID HOT AQUEOUS SALT SOLUTION TO FURTHER INCREASE THE REFRACTORINESS OF THE MOLD; THE SOLUTE OF SAID DETERGENT SOLUTION BEING A MATERIAL SELECTED FROM THE GROUP CONSISTIG OF ALKALINE ALUMINATE, ALKALINE ZINCATE, ALKALINE ZIRCONATE, ALKALINE TITANATE, AND ALKALINE SILICATE; AND THEN HEAT DRYING THE MOLD. 